This invention relates to radiation cured conjugated diene-vinyl aromatic hydrocarbon block copolymers which are cured at low radiation doses. More particularly, the invention relates to improved crosslinking of such epoxidized polymers.
Curing of adhesives based on conjugated diolefins and vinyl aromatics has increased the range of service properties for such adhesives. Radiation curing of polymers to make such adhesives is known. Curing of coatings and sealants based on these polymers has reduced solvent content required for such products. This curing causes covalent crosslinking of the polymerized conjugated diolefins which is evidenced by a high gel content of the crosslinked polymer. Before crosslinking, the polymers are melt and solution processable but after crosslinking, the gel cannot be processed as melts or in solution. Crosslinking therefore enhances solvent resistance and improves elevated temperature shear properties, toughness and cohesion. Compositions can therefore be applied to a substrate in a melt or from solution and then crosslinked to form a superior adhesive, coating or sealant. However, improvements in the adhesives, coatings and sealants could be made if the adhesives could be cured at lower dosages of radiation.
Epoxidized block copolymers of conjugated diolefins and vinyl aromatic hydrocarbons have been disclosed as being useful in radiation cured adhesive, coating and sealant compositions (see U.S. Pat. No. 5,229,464). Such polymers were said to be curable using electron beam and ultraviolet radiation. It has been found that epoxidized polymers of conjugated dienes which also contain aromatic moieties require much more irradiation when ultraviolet radiation is used to cure the polymers than such polymers which do not contain aromatic moieties. Such aromatic moieties may be provided by a vinyl aromatic hydrocarbon monomer which is copolymerized with the conjugated diene or they may be provided when an aromatic coupling agent such as divinyl benzene is used to couple polymer arms together.
Radiation curing of such polymers is not an inexpensive process. Increasing the amount of irradiation necessary to cure a polymer increases the overall cost of producing the product. Therefore, it would be advantageous to provide a method wherein epoxidized diene polymers which contain aromatic moieties could be cured to the same extent with approximately the same amount of irradiation as is necessary to cure epoxidized diene polymers which do not contain aromatic moieties. The present invention provides such a process.